Acoustic apparatus



July 30, 1929,

E. C. WENTE ACOUSTIC APPARATUS Filed Oct. 20, 1926 L5 2'/ A? 22 i3 20 )2Patented July' 30, 1929.

UNITED STATES than? oFFme.

EDWARD C. WEN'IE, OF NEW YORK, N. Y., ASSIGNOR TO BELL TELEPHONELALORA'IO- RIES, INCORPORATED, OF NEW YORK, N. Y., A EORPOB ATION OF NEWYORK.

ACOUSTIC APPARATUS.

Application filed October 20, 1926. Serial No. 142,790.

This invention relates to acoustic appa ratus and particularly totelephone transmitters.

An object of the invention is to improve the quality of the transmissioncharacteristic of such instruments.

Transmitters of the type to which this invention relates consistessentially of a rigid back plate and a flexible diaphragm mounted inclose space relation thereto. In some transmitters the plate servesessentially to damp the motion of the diaphragm while in those of thecondenser type it 'also serves as one electrode. In order to preventundue damping at the high frequencies produced by friction of the thinlayer of air separating the plate and the diaphragm, it has been founddesirable to provide grooves in the face of the back plate and toperforate the :0 plate as disclosed and claimed in U. S. Patent No.1,456,538 granted to I. B. Crandall May 29, 1923. As heretoforeconstructed these grooves have been found to give resonance effectswhich in many cases impair the quality of the transmissioncharacteristics of the instrument. It also has heretofore been founddesirable to make the transmitter in the form of a gas-tight chamberhaving a front wall formed by the diaphragm and a rear wall formed by asheet of flexible material on the opposite side of the perforated backplate for equalizing the pressure within and without the chamber. Inaddition to equalizing the pressure such a flexible wall alsoselectively transmits sound waves, which action is undesirable, as itcauses the impedance offered to the diaphragm to vary with frequency. v

In a preferred embodiment of this invention two chambers are employed.One has its front wall formed by the diaphragm, contains the back plateand includes a small space therebehind. The other has a flexible wallfor equalizing the pressure within the chambers with that of theatmosphere without materially displacing the diaphragm, and is connectedto the front chamber by an 'air passage which is opaque to waves ofspeech andmusic frequencies. The grooves in the front plate are made ina grid formation to give a maximum air passage with a minimum cuttingaway of material and are 1 closed at the edges of the plate to preventresonance in the portion of the chamber surrounding the plate. Thechambers are filled with inert gas to prevent corrosion and reduction111 pressure due to the consumption of the air within the chamber by theoxidation of the sealing compound.

One type of air passage which has been found very suitable forconnecting the chambers consists of a pieces of silk or other fabricheld in place by a metallic sheet so that the air can pass through thefabric only in a longitudinal direction under the edges of the metallicsheet.

It should be understood that the various features of this preferredembodiment may be used separately or in subcombinations withoutdeparting from the spirit of the invention as defined in the appendedclaims.

This invention can be more readily understood by reference to thefollowing detailed description in connection with the drawings in which:

Fig. 1 shows an elevation partly in section of a condenser transmitterconstructed according'to this invention;

Fig. 2 shows the face of the back plate of the transmitter; and

Fig. 3 shows in detail the arrangement of the grooves in the face of theback plate.

There is provideda main frame member 4 in which the remaining parts ofthe transmitter are mounted. The flexible diaphragm 5 is made ofduralium, as disclosed and claimed in Patent No. 1,611,870 to H. 'A.Anderson, issued Dec. 28, 1926. This diaphragm is clamped between twothin duralium rings 6, against a shoulder of the frame member 4 by meansof the rings 7 and 8, the latter of which is screwed in the threads onthe inner surface of the frame member 4. The diaphragm 5 is stretched bymeans of a threaded ring 9 which is screwed into the end of the framemember 4 opposite to that into which the ring 8 is screwed.

' The back plate and associated members are assembled on a ring 10, oneend of which rests against the diaphragm and the other end of which isclamped in the frame member 4 by means of a threaded ring 11. The backplate consists of a central grooved and perforatedportion 12 and anouter ring 13 which is pressed th'ereover to close the outer ends of 5the grooves. The details of the grooves, which are best shown in Figs. 2and 3, will be described hereinafter. The back plate is mounted withinthe ring 10, being separated therefrom by an insulating was er 14 andbeing clamped therein by means of a threaded ring 15, which is clampedagainst a metallic washer 16 which is in turn separated from the backplate by an insulating washer 17. A piece of silk 18 covers theperforations in the back plate to prevent dust from getting into thespace between the diaphragm and the back plate during assembly. Thechamber 19 behind the back plate is closed by a metallic plate 20 havinga hole 21 in the center thereo piece of silk 22 clamped thereover by asecond plate 23. A flexible duralium diaphragm24 is clamped between theplate 20 and a ring 25 by means of a threaded ringv26 which screws intothe threads on the inside of the back plate member12'. A terminal 27 isprovided for making connection to the back plate and sealing compound 30is provided for making the entire assembly gas tight. The edges of therear diaphragm 24 are also painted with sealing compound beforeassembly. A back insulating plate 29 is secured to the ring 26 toenclose the insulated parts. Before the insulating plate 29 is put inplace, the air behind the diaphragm 5 is pumped out through a hole 31extending through the members 12 and 13, the space is filled with aninert gas such as nitrogen and the outer end of the hole 31 is sealed.

As shown in Figs. 2 and 3 the grooves 32 in the face of the back plateare arranged in grid formation being formed by straight grooves at rightangles to one another, and

holes 33 are bored through the plate at the intersection of the grooves.These holes are tapered toreduce the resonant effects which they tend tointroduce. In order to prevent resonant effects which might be producedif the grooves communicated with the portion of chamber surrounding theback plate, the outer ends of the grooves are closed by the annularplate 13.

The rear diaphragm 24 is provided to equalize the pressure in thechamber 19, behind the main diaphragm 5, with that of the atmosphere toprevent material displacenients of the main diaphragm due to changes inthe atmospheric pressure. The only gas passage from the rear chamber,which is closed by the diaphragm 24, to the chamber 19 is through thehole 21 and longitudinally through the silk 22 so that no waves ofspeech or music frequencies can be transmitted between the chamber 19and the diaphragm 24 while gas may slowly pass from one chamber to theother to equalize the pressure.

What is claimed is:

1. An acoustic device comprising a rigid plate, a flexible diaphragm infront of said plate, a chamber containing said plate, a second chamber,an air passage substantially opaque to waves of speech and musicfrequencies connecting said chambers, and a flexible wall for saidsecond chamber.

This hole is covered by a 2. An acoustic device according. to thepreceding claim in which the air passage comprises a piece of fabric anda solid plate tially opaque to waves of speech and music frequenciesconnecting said chambers, a rigid plate in one of said chambers, a,flexible acoustic diaphragm in close space relation to said plate andforming a wall of said chamber,

and a flexible wall for the other of said chambers whereby the pressurewithin said chambers is equalized with that of the atmosphere withoutmaterially displacing said diaphragm.

4. An acoustic device comprising a rigid plate, a flexible diaphragm infront of said plate, a chamber having a wall formed by said diaphragm,and inert gas substantially filling said chamber. g

5. An acoustic device comprising a diaphragm and a back plate in closespace relation thereto, having grooves in the face thereof arranged in agrid pattern and perforations at the intersections of the grooves.

6. An acoustic device comprising a diaphragm, a rigid plate in closespace relation thereto having grooves therein on one side of said plate,said grooves being closed at their outer ends. r

7. An acoustic device comprising a flexible diaphragm, a rigid groovedplate in close space relation thereto, and means closing said grooves atthe edge of the plate.

8. An acoustic device comprising, a stretched metallic diaphragm and aback plate mounted in close space relation thereto and having grooves inthe face thereof arranged in a grid pattern to prevent undue damping atthe higher frequencies of the speech and music range. Y

9. An acoustic device according to the preceding claim in which thegrooves are closed at the edge of the plate.

10. A transmitter comprising a stretched metallic diaphragm, a rigidback plate mounted in close space relation thereto, having grooves inthe face thereof arranged in a grid pattern and perforations at theintersections of the grooves, means for closing said grooves at theirouter ends, a chamber perforated rigid plate adjacent said member, apair of chambers on one side of said plate,

one of said chambers having a flexible wall, said chambers having a wallbetween them comprising two plates, one of which has an opaque to thepassage of waves of speech or opening therein anda fabric member betweenmusic frequencies from one chamber to an- 10 said plates. other butwhich will permit the passage of 12. A transmitter comprising avibratable gas from one chamber to another to equalize 5 member, aplurality of chambers on one side the pressure in the chambers.

of said member, one of which is expansible,-a In witness whereof, Ihereunto subscribe filling of gas in one of said chambers, and a my namethis 19th da of October A. D. 1926.

passage between said chambers. which is ED ARD C. WENTE.

